Positive electrode active material, positive electrode for nonaqueous electrolyte battery, and nonaqueous electrolyte battery

ABSTRACT

The present invention provides a nonaqueous electrolyte battery that exhibits high energy density and excellent cycle characteristics, as well as a cathode for use in such a battery, and a cathode active material for use in such a cathode. The cathode active material of the present invention has a composition represented by the formula (1) and a crystallite size in the (110) plane of not smaller than 85 nm: 
       Li x Co 1-y-z Nb y M z O 2   (1) 
     wherein M stands for at least one element selected from Mg, Y, rare earth elements, Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Ni, Cu, Zn, B, Al, Ga, C, Si, Sn, N, S, F, and Cl; and 0.9≦x≦1.1, 0.0002≦y≦0.01, and 0≦z≦0.05.

FIELD OF ART

The present invention relates to a cathode active material fornonaqueous electrolyte batteries, such as lithium ion rechargeablebatteries, a cathode for nonaqueous electrolyte batteries, and anonaqueous electrolyte battery.

BACKGROUND ART

Lithium ion rechargeable batteries as a nonaqueous electrolyte batteryare widely used in portable electronic devices, such as video cameras,mobile phones, and notebook computers, which have been becoming smaller,lighter, and more powerful. The batteries are also expected to be usedin hybrid and electronic vehicles. For such applications, it is recentlyan important problem to improve the energy density of the batteries.

In order to solve this problem, it is conceivable to increase themaximum charging voltage and the discharge voltage. For lithium ionrechargeable batteries employing LiCoO₂ as a cathode active material,the current maximum charging voltage is about 4.2 V vs lithium. At ahigher charging voltage of beyond 4.2 V, the amount of deintercalatedlithium is excessive, which increases crystal strain of the cathodeactive material, resulting in collapse of the crystal structure. Thus,it is necessary not only to raise the maximum charging voltage, but alsoto improve stability of the crystal structure in the charged state, forpreventing deterioration of discharge capacity and cyclecharacteristics.

For improving stability at high voltage of the crystal structure ofLiCoO₂ as a cathode active material, it is proposed to substitute partof Co with one or more different elements. For example, PatentPublication 1 proposes a cathode active material wherein part of Co issubstituted with Mg and M representing at least one element selectedfrom the group consisting of Al, Ti, Sr, Mn, Ni, and Ca. PatentPublication 2 proposes a cathode active material wherein part of Co issubstituted with elements of groups IV-A and II-A. Patent Publication 3proposes a cathode active material that withstands high voltage and hashigh capacity and excellent cycle characteristics, wherein part of Co issubstituted with A representing at least one element selected from thegroup consisting of Ti, Ta, and Nb, and B representing at least oneelement selected from the group consisting of Al, Fe, Ni, Y, Zr, W, Mn,In, Sn, and Si. Patent Publication 4 proposes a cathode active materialthat has high capacity and excellent low-temperature properties, whereinpart of Co may be substituted with M representing at least one elementselected from the group consisting of Ta, Ti, Nb, Zr, and Hf.

The proposed cathode active materials, however, do not have sufficientstability of LiCoO₂ crystal structure. Batteries having high energydensity and excellent cycle characteristics are yet to be obtained.

Patent Publication 1: JP-2004-220952-A Patent Publication 2:JP-2005-50779-A Patent Publication 3: JP-2001-351624-A PatentPublication 4: WO-01-027032-A SUMMARY OF THE INVENTION

It is an object of the present invention to provide a nonaqueouselectrolyte battery that exhibits, even at a maximum charging voltage ofabout 4.2 V, high energy density and excellent cycle characteristics,and at an increased maximum charging voltage of about 4.6 V, exhibitshigh energy density and still more excellent cycle characteristics, aswell as a cathode for the nonaqueous electrolyte battery and a cathodeactive material for the cathode.

It is another object of the present invention to provide a nonaqueouselectrolyte battery that exhibits high energy density, excellent cyclecharacteristics, and also excellent thermal stability, as well as acathode for the nonaqueous electrolyte battery and a cathode activematerial for the cathode.

According to the present invention, there is provided a cathode activematerial of a composition represented by the formula (1) having acrystallite size in the (110) plane of not smaller than 85 nm:

Li_(x)Co_(1-y-z)Nb_(y)M_(z)O₂  (1)

wherein M stands for at least one element selected from Mg, Y, rareearth elements, Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Ni, Cu, Zn, B, Al,Ga, C, Si, Sn, N, S, F, and Cl; and 0.9≦x≦1.1, 0.0002≦y≦0.01, and0≦≦z≦0.05.

According to the present invention, there is also provided a cathode fora nonaqueous electrolyte battery comprising the above cathode activematerial.

According to the present invention, there is also provided a nonaqueouselectrolyte battery comprising the above cathode for a nonaqueouselectrolyte battery.

According to the present invention, there is further provided use of theabove cathode active material in the manufacture of a cathode for anonaqueous electrolyte battery.

Utilizing the cathode active material having the above-mentionedstructure, the nonaqueous electrolyte battery and the cathode for thenonaqueous electrolyte battery according to the present inventionexhibit high energy density and excellent cycle characteristics. Thecathode active material according to the present invention is extremelyuseful in production of a cathode for nonaqueous electrolyte batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the X-ray diffraction spectrum of the cathodeactive material prepared in Example 1.

FIG. 2 is a copy of a SEM image at ×5000 magnification of the cathodeactive material prepared in Example 1.

FIG. 3 is a graph showing the X-ray diffraction spectrum of the cathodeactive material prepared in Comparative Example 3.

FIG. 4 is a copy of a SEM image at ×5000 magnification of the cathodeactive material prepared in Comparative Example 3.

FIG. 5 is a graph showing the results of differential scanningcalorimetry of the cathode active materials prepared in Example 1,Example 5, and Comparative Example 5.

PREFERRED EMBODIMENTS OF THE INVENTION

The present invention will now be explained in more detail.

The cathode active material according to the present invention has acomposition represented by the formula (1) mentioned above.

In the formula (1), M stands for at least one element selected from Mg,Y, rare earth elements, Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Ni, Cu,Zn, B, Al, Ga, C, Si, Sn, N, S, F, and Cl.

In the formula (1), x denotes the amount of Li upon synthesis of thecathode active material, and is 0.9≦x≦1.1. With x within this range, thecathode active material is given a LiCoO₂ single phase structure. When abattery is produced with the cathode active material and subjected tocharging/discharging, the amount of Li varies due to intercalation anddeintercalation.

In the formula (1), y denotes the amount of Nb, and is 0.0002≦y≦0.01. Itis not known in detail how Nb is present and functions in the cathodeactive material, but substituting part of Co with Nb and making thecrystallite size in the (110) phase fall within the particular range aswill be discussed later, stabilize the crystal structure. Thus, usingsuch a cathode active material in a cathode of a nonaqueous electrolytebattery, high energy density and excellent cycle characteristics may beachieved even at a maximum charging voltage of about 4.2 V. Further, atan increased maximum charging voltage of about 4.6 V, still moreexcellent energy density and cycle characteristics may be achieved.

In the cathode active material according to the present invention, it ispreferred that Nb is present in such a state that no peak correspondingto the secondary phase (an oxide of Nb or a Li—Nb composite oxide) isobserved in X-ray diffraction. Further, it is preferred that unevendistribution of Nb in a cross-section of the cathode active material isnot observed with EPMA (Electron Probe Micro Analyzer) at ×1000magnification. If y is less than 0.0002, the effect of Nb to stabilizethe crystal structure is not exhibited sufficiently. If y is more than0.01, the secondary phase is precipitated, the effect of Nb to stabilizethe crystal structure is not exhibited sufficiently, the capacity islowered, and the internal resistance is increased. y is preferably0.0005≦y<0.005, more preferably 0.001≦y<0.005. With y within theseranges, no peak corresponding to the secondary phase is observed inX-ray diffraction, and uneven distribution of Nb in a cross-section isnot observed with EPMA, which result in stabilized crystal structure andhigh capacity.

In the formula (1), M stands for one or more elements selected from Mg,Y, rare earth elements, Ti, Zr, Hf, V, Ta, Cr, Mo, W, Mn, Fe, Ni, Cu,Zn, B, Al, Ga, C, Si, Sn, N, S, F, and Cl. For example, when M is Mg,thermal stability is greatly improved. Addition of Mg as M, compared toaddition of only Nd, results in a synergistic effect that the averagedischarge voltage is increased at a higher maximum charging voltage.

z denotes the amount of M, and is 0≦z≦0.05. The cathode active materialaccording to the present invention does not necessarily contain M.However, M may be contained for the purpose of improving various batteryproperties, or as inevitable impurities. If z is more than 0.05, thesecondary phase is precipitated, which causes decrease in capacity. Thebalance of various battery properties being considered, it is preferredthat M is Mg and z is 0.005≦z≦0.02.

The cathode active material according to the present invention has acrystallite size in the (110) plane of not smaller than 85 nm.

In the present invention, the crystallite size in the (110) plane iscalculated according to the Scherrer formula from the peak near2θ=66.5±1° in X-ray diffraction spectrum determined by means of an X-raydiffractometer (RINT2000 manufactured by RIGAKU CORPORATION) with CuKαradiation. With the crystallite size in the (110) plane of not smallerthan 85 nm, the crystal structure is stable. Addition of Nb results inthe tendency for the crystallite size in the (110) plane to decrease dueto suppressed growth of the primary particles, so that it is necessaryto control the starting materials and manufacturing conditions forachieving the size of not smaller than 85 nm. If the crystallite size inthe (110) plane is less than 85 nm, the crystal structure is unstableduring charging, which deteriorates discharge capacity and cyclecharacteristics.

The cathode active material according to the present invention may beprepared, for example, by mixing a Li compound as a Li source, a Cocompound as a Co source, a Nb compound as a Nb source, and optionally anM compound as an M source, and calcining the resulting mixture underappropriate conditions.

Examples of the Li compound may include inorganic salts, such as lithiumhydroxide, lithium chloride, lithium nitrate, lithium carbonate, andlithium sulfate; and organic salts, such as lithium formate, lithiumacetate, and lithium oxalate.

Examples of the Co compound may include oxide, hydroxide, carbonate, andoxyhydroxide of cobalt. Among these, cobalt oxide is preferred, andparticularly preferred is cobalt oxide in the form of spheres having anaverage primary particle size of 50 to 200 nm and an average secondaryparticle size of 5 to 20 μM. Use of such spherical oxide particles as astarting material remarkably improves the reactivity with Nb to suppresssegregation of Nb in the particles, and to achieve the crystallite sizein the (110) plane of not smaller than 85 nm.

The oxide in the form of spheres may be prepared, for example, byintroducing an aqueous solution of a Co compound and an alkaline aqueoussolution into a reaction vessel under stirring at constant temperatureand pH to prepare a hydroxide in the form of spheres, and calcining thehydroxide.

In the preparation of the hydroxide, a complexing agent, such as anammonium salt, may suitably be added into the reaction vessel.

The calcination of the spherical hydroxide thus obtained may beperformed usually at 300 to 800° C. for 1 to 24 hours. This step mayalso be performed by preliminary calcination at a temperature lower thanthe intended temperature, followed by raising up to the intendedtemperature, or by calcination at the intended temperature, followed byannealing at a lower temperature.

The sizes of the primary and secondary particles of the spherical oxidemay easily be controlled by adjusting the concentration of the aqueoussolution of a Co compound, the concentration of the alkaline aqueoussolution, the adding rate of these solutions, the pH and temperature inthe reaction vessel, the concentration of the complexing agent, as wellas the conditions of calcination of the resulting hydroxide.

In this way, the oxide in the form of spheres having an average primaryparticle size of 50 to 200 nm and an average secondary particle size of5 to 20 μm may be obtained, which is suitable as a Co source.

Examples of the Nb compound may include niobium oxide, preferably Nb₂O₅.Since the content of Nb is a trace amount, it is necessary to uniformlydisperse Nb in mixing with the Li compound and the Co compound.Otherwise, Nb may be segregated or the secondary phase may precipitateafter the calcination. For improved dispersibility, the average particlesize of the Nb compound is preferably 1 to 5 μm.

Examples of the M compound may include, though varying depending on theselected element, oxides, hydroxides, carbonates, sulfates, nitrates,and halides containing M, and gases containing M.

For preparing the cathode active material of the present invention,first, for example, the Li compound, the Co compound, the Nb compound,and optionally the M compound are respectively measured out and mixed.The mixing may be performed in a ball mill or the like according to aknown method, but it is preferred to use a high-speed stirring mixer inorder to improve dispersibility.

Next, the mixture thus obtained is calcined. The calcination may beperformed in a bogie hearth furnace, a kiln furnace, a mesh beltfurnace, or the like, according to a known method, usually at higherthan 1000° C. for 1 to 24 hours. Preferred calcination temperature is1030 to 1050° C. At 1000° C. or lower, the crystallite size in the (110)plane may not be ensured to be not smaller than 85 nm.

The calcination may also be performed by preliminary calcination at atemperature lower than the above-mentioned calcination temperature,followed by raising up to the calcination temperature, or by calcinationat the above-mentioned calcination temperature, followed by annealing ata lower temperature. The preliminary calcination or the annealing may beperformed usually at 500 to 800° C. for about 30 minutes to about 6hours.

The cathode active material according to the present invention mayalternatively be prepared by mixing a Li compound and a compositecompound prepared by coprecipitation of Co, Nb, and optionally M, andcalcining the resulting mixture.

The cathode for a nonaqueous electrolyte battery according to thepresent invention contains the cathode active material of the presentinvention discussed above.

The cathode of the present invention may be prepared by a known method,using the cathode active material of the present invention as a cathodeactive material. For example, the cathode may be prepared by mixing thecathode active material, an electrically conductive material, a binder,and the like with an organic solvent, applying the resulting paste to acollector, drying, rolling, and cutting into a predetermined size. Theelectrically conductive material, the binder, the organic solvent, andthe collector may be known products.

Examples of the electrically conductive material may includecarbonaceous materials, such as natural graphite, artificial graphite,Ketjen black, and acetylene black.

Examples of the binder may include fluororesins, such aspolytetrafluoroethylene and polyvinylidene fluoride, polyvinyl acetate,polymethyl methacrylate, ethylene-propylene-diene copolymer,styrene-butadiene copolymer, acrylonitrile-butadiene copolymer, andcarboxymethyl cellulose.

Examples of the organic solvent may include N-methylpyrrolidone,tetrahydrofuran, ethylene oxide, methyl ethyl ketone, cyclohexanone,methyl acetate, methyl acrylate, diethyltriamine, dimethylformamide, anddimethylacetamide.

Examples of the collector may include metal foils, such as Al, Cu, andstainless steel foils.

The cathode active material used in the cathode of the present inventionmay be the cathode active material of the present invention mixed with aknown cathode active material for achieving desired battery properties.For example, a cathode active material composed mainly of Ni, such asLiNiO₂, may be admixed to improve the discharge capacity, orLiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ may be admixed to improve safety.

The nonaqueous electrolyte battery according to the present invention isprovided with the cathode for a nonaqueous electrolyte battery accordingto the present invention.

The constituent components of the nonaqueous electrolyte battery of thepresent invention other than the cathode may be of known structures. Thenonaqueous electrolyte battery of the present invention is composedmainly of, for example, a cathode, an anode, an organic solvent, anelectrolyte, and a separator. The organic solvent and the electrolytemay be replaced with a solid electrolyte.

In the anode, an anode active material is contained, such as a lithiummetal, a lithium alloy, or a carbonaceous material. A binder, acollector, and the like, which are similar to those for the cathode, areoptionally used. Examples of the carbonaceous material may includeamorphous carbon, such as soft carbon and hard carbon, artificialgraphite, and natural graphite.

Examples of the organic solvent may include carbonates, such aspropylene carbonate, ethylene carbonate, dimethyl carbonate, diethylcarbonate, and ethyl methyl carbonate; ethers, such as1,2-dimethoxypropane, 1,3-dimethoxypropane, tetrahydrofuran, and2-methyltetrahydrofuran; esters, such as methyl acetate andγ-butyrolactone; nitriles, such as acetonitrile and butylonitrile; andamides, such as N,N-dimethylformamide and N,N-dimethylacetamide.

Examples of the electrolyte may include LiClO₄, LiPF₆, and LiBF₄.

Examples of the solid electrolyte may include polymer electrolytes, suchas polyethylene oxide electrolyte; and sulfate electrolytes, such asLi₂S—SiS₂, Li₂S—P₂S₅, and Li₂S—B₂S₃. Alternatively, a so-called gel-typeelectrolyte, wherein a nonaqueous electrolyte solution is retained in apolymer, may also be used.

Examples of the separator may include porous polymer membranes, such asof polyethylene or polypropylene.

The nonaqueous electrolyte battery according to the present inventionmay take various shapes, such as cylindrical, laminated, and coinshapes. In any shape, the nonaqueous electrolyte battery of the presentinvention may be fabricated by placing the above-mentioned constituentcomponents in a battery case, connecting the cathode and the anode to acathode terminal and an anode terminal, respectively, with collectorleads, and sealing the battery case.

EXAMPLES

The present invention will now be explained in more detail withreference to Examples and Comparative Examples, which are not intendedto limit the present invention.

Example 1

500 ml of an aqueous solution of cobalt sulfate with a Co content of 1mol/l and 1000 ml of a 1 mol/l aqueous solution of sodium hydroxide wereintroduced into a reaction vessel under stirring so as to be at 50° C.and pH 8 to 12. After the introduction, the mixture was stirredcontinuously, and maintained at 50° C. for 10 hours for ageing. Then theresulting precipitate was taken out by filtration, and calcined in abox-shaped electric furnace at 300° C. for 5 hours to obtain cobaltoxide in the form of spheres. The major axis of more than 100arbitrarily-selected primary particles of the spherical cobalt oxidethus obtained was measured using a ×5000 SEM image. It was determinedthat the average primary particle size represented as an average valueof the major axis was 100 nm. The average secondary particle size (D50)determined by laser diffraction was 16 μm.

The spherical cobalt oxide prepared above, lithium carbonate, andniobium oxide having an average particle size (D50) of 2 μm weremeasured out at a ratio of Li:Co:Nb=1:0.9990:0.0010, and mixed in ahigh-speed stirring mixer. The resulting mixture was calcined in abox-shaped electric furnace at 1010° C. for 6 hours to obtain a cathodeactive material. The X-ray diffraction spectrum of this cathode activematerial determined with CuKα radiation is shown in FIG. 1.

The result was that only a diffraction peak corresponding to thehexagonal system was observed. The crystallite size determined from thediffraction peak in the (110) plane was 87.3 nm. The cathode activematerial was observed under SEM at ×5000 magnification. A copy of theSEM image is shown in FIG. 2. Further, a cross-section of the cathodeactive material was observed with EPMA at ×1000 magnification to confirmthat Nb was not unevenly distributed in any particles.

The cathode active material thus obtained, acetylene black as anelectrically conductive material, and polyvinylidene fluoride as abinder were mixed at a ratio of 93:2:5 by mass, and kneaded withN-methylpyrrolidone into a paste. The paste was applied onto aluminumfoil of 20 μm thick, dried, and pressure molded in a press. Theresulting product was cut into a predetermined size, and terminals werespot-welded thereto to produce a cathode. On the other hand, lithiumfoil was fixed to stainless steel mesh by pressing, and terminals werespot-welded thereto to produce an anode. An electrode prepared in thesame way as the anode was used as a reference electrode. Theseelectrodes were placed in a glass container with their terminalsprojecting from each electrode, and an electrolyte prepared bydissolving lithium perchlorate in a 1:1 (by volume) mixture of ethylenecarbonate and ethyl methyl carbonate at 1 mol/l, was introduced into thecontainer to produce a nonaqueous electrolyte battery.

The battery was subjected to charging/discharging at 1 C (C=150 mA/g)between the maximum charging voltage of 4.3 V and the minimumdischarging voltage of 3V vs the reference electrode. Then the dischargecapacity and the average discharge voltage per 1 g of the cathode activematerial at the first cycle were taken as the initial capacity and theinitial discharge voltage, respectively. It was determined that theinitial capacity of this battery was 152 mAh/g, and the initialdischarge voltage was 3.84 V. The discharge rate at the 20th cycle ofcharging/discharging under the same conditions, divided by the initialcapacity, was taken as a cycle capacity characteristics value.Similarly, the average discharge voltage at the 20th cycle divided bythe initial discharge voltage was taken as a cycle voltagecharacteristics value. The cycle capacity characteristics value of thisbattery was 96.2% and the cycle voltage characteristics value was 98.0%.The results are shown in Table 2.

On the other hand, the initial capacity, the initial discharge voltage,the cycle capacity characteristics value, and the cycle voltagecharacteristics value of the battery were determined by performingcharging/discharging under the same conditions as above, except that themaximum charging voltage was 4.5 V. In this case, the initial capacitywas 180 mAh/g, the initial discharge voltage was 3.92 V, the cyclecapacity characteristics value was 83.9%, and the cycle voltagecharacteristics value was 88.5%. The results are shown in Table 3.

The average discharge voltage at the second cycle ofcharging/discharging under the same conditions as above, except that themaximum charging voltage was 4.6 V, was taken as a 4.6 V averagedischarge voltage. The 4.6 V average discharge voltage of this batterywas 3.93 V. The result is shown in Table 1.

The cathode was taken out from the battery that had been charged to 4.3V, and cut into a predetermined size to prepare a cathode piece. Thiscathode piece was sealed in an aluminum measuring cell together with theelectrolyte discussed above. The cell was pierced for degassing. Themeasuring cell thus produced was subjected to differential scanningcalorimetry in a DSC (differential scanning calorimetry) system at thetemperature-raising rate of 5° C./min from the room temperature up to300° C. The results are shown in FIG. 5.

Examples 2-4

A cathode active material, a cathode, and a nonaqueous electrolytebattery were prepared in the same way as in Example 1, using cobaltoxide in the form of spheres, lithium carbonate, and niobium oxide ateach composition shown in Table 1. The crystallite size in the (110)plane of the obtained cathode active material and the battery propertiesof the nonaqueous electrolyte battery thus produced were determined inthe same way as in Example 1. The results are shown in Tables 1 to 3.

Examples 5 and 6

A cathode active material, a cathode, and a nonaqueous electrolytebattery were prepared in the same way as in Example 1, using cobaltoxide in the form of spheres, lithium carbonate, niobium oxide, andmagnesium hydroxide at each composition shown in Table 1. The magnesiumhydroxide used here had an average particle size (D50) of 4 μm.

The crystallite size in the (110) plane of the obtained cathode activematerial and the battery properties of the nonaqueous electrolytebattery thus produced were determined in the same way as in Example 1.The results are shown in Tables 1 to 3. The results of differentialscanning calorimetry of the cathode active material prepared in Example5 are shown in FIG. 5. It was confirmed that addition of Mg caused theexothermic peak to generally shift to the higher-temperature side,indicating improvement in thermal resistance.

Comparative Examples 1-3

A cathode active material, a cathode, and a nonaqueous electrolytebattery were prepared in the same way as in Example 1, using cobaltoxide in the form of spheres, lithium carbonate, and niobium oxide ateach composition shown in Table 1. The crystallite size in the (110)plane of the obtained cathode active material and the battery propertiesof the nonaqueous electrolyte battery thus produced were determined inthe same way as in Example 1. The results are shown in Tables 1 to 3.

The X-ray diffraction spectrum of the cathode active material preparedin Comparative Example 3 are shown in FIG. 3, wherein a diffraction peakcorresponding to Nb oxide was confirmed in addition to the peakcorresponding to the hexagonal system. Further, the cathode activematerial prepared in Comparative Example 3 was observed under SEM at×5000 magnification. A copy of the SEM image is shown in FIG. 4, whereinprecipitate, which is believed to be Nb oxide, was observed.

Comparative Example 4

A cathode active material, a cathode, and a nonaqueous electrolytebattery were prepared in the same way as in Example 1, except that thecalcination of the mixture at 1010° C. for 6 hours in Example 1 waschanged to calcination at 900° C. for 6 hours. The crystallite size inthe (110) plane of the obtained cathode active material and the batteryproperties of the nonaqueous electrolyte battery thus produced weredetermined in the same way as in Example 1. The results are shown inTables 1 to 3.

Comparative Example 5

A cathode active material, a cathode, and a nonaqueous electrolytebattery were prepared in the same way as in Example 1, using cobaltoxide in the form of spheres, lithium carbonate, and magnesium hydroxideat the composition shown in Table 1, except that the calcination of themixture at 1010° C. for 6 hours in Example 1 was changed to calcinationat 990° C. for 6 hours. The magnesium hydroxide used here had an averageparticle size (D50) of 4 μm.

The crystallite size in the (110) plane of the obtained cathode activematerial and the battery properties of the nonaqueous electrolytebattery thus produced were determined in the same way as in Example 1.The results are shown in Tables 1 to 3. The results of differentialscanning calorimetry are shown in FIG. 5. From FIG. 5, it was found thatrising of an exothermic peak was observed at approximately the sametemperature as in Example 5.

From the results of measurements in Examples and Comparative Examples,it is understood that control of the crystallite size in the (110)plane, in addition to the content of Nb, to fall within the rangeaccording to the present invention resulted in improvement in variousbattery properties at the maximum charging voltage of 4.3 V, moreover at4.5 V or 4.6 V. On the other hand, it is also understood that control ofthe Nb content, in addition to the crystallite size in the (110) plane,to fall within the range according to the present invention resulted inimprovement in various battery properties at the maximum chargingvoltage of 4.3 V, moreover at 4.5 V or 4.6 V. It is further understoodthat addition of Mg together with Nb resulted in improvement in not onlyvarious battery properties but also thermal stability at the maximumcharging voltage of 4.3 V or 4.5 V, moreover at 4.6 V.

TABLE 1 4.6 V Average Crystallite discharge Nb Mg size voltage Li Co(value of y) (value of z) (Å) (V) Example 1 1 0.9990 0.0010 0 873 3.93Example 2 1 0.9995 0.0005 0 911 3.87 Example 3 1 0.9900 0.0100 0 8533.92 Example 4 1 0.9960 0.0040 0 864 3.93 Example 5 1 0.9890 0.00100.0100 1008 3.97 Example 6 1 0.9490 0.0010 0.0500 890 3.97 Comp. Ex. 1 10.9999 0.0001 0 1046 3.80 Comp. Ex. 2 1 1.0000 0 0 1038 3.82 Comp. Ex. 31 0.9800 0.0200 0 791 3.84 Comp. Ex. 4 1 0.9990 0.0010 0 840 3.81 Comp.Ex. 5 1 0.9900 0 0.0100 917 3.79

TABLE 2 Cycle capacity Cycle voltage Initial characteristics Initialcharacteristics capacity value voltage value (mAh/g) (%) (V) (%) Example1 152 96.2 3.84 98.0 Example 2 149 95.7 3.84 97.5 Example 3 141 94.83.83 96.6 Example 4 149 95.3 3.84 97.1 Example 5 150 94.5 3.91 96.8Example 6 140 94.8 3.84 95.9 Comp. Ex. 1 148 90.0 3.80 91.7 Comp. Ex. 2149 91.3 3.82 92.9 Comp. Ex. 3 129 93.5 3.82 95.2 Comp. Ex. 4 147 90.53.80 92.2 Comp. Ex. 5 148 93.7 3.83 95.3

TABLE 3 Cycle capacity Cycle voltage Initial characteristics Initialcharacteristics capacity value voltage value (mAh/g) (%) (V) (%) Example1 180 83.9 3.92 88.5 Example 2 177 80.2 3.91 84.6 Example 3 168 82.93.91 87.0 Example 4 177 83.5 3.92 87.7 Example 5 177 82.3 3.91 87.5Example 6 166 80.3 3.91 83.5 Comp. Ex. 1 175 70.2 3.88 74.0 Comp. Ex. 2176 65.3 3.89 79.4 Comp. Ex. 3 152 72.3 3.89 75.9 Comp. Ex. 4 173 71.53.88 75.1 Comp. Ex. 5 174 74.8 3.83 79.7

1. A cathode active material of a composition represented by the formula(1) having a crystallite size in the (110) plane of not smaller than 85nm:Li_(x)Co_(1-y-z)Nb_(y)M_(z)O₂  (1) wherein M stands for at least oneelement selected from Mg, Y, rare earth elements, Ti, Zr, Hf, V, Ta, Cr,Mo, W, Mn, Fe, Ni, Cu, Zn, B, Al, Ga, C, Si, Sn, N, S, F, and Cl; and0.9≦x≦1.1, 0.0002≦y≦0.01, and 0≦z≦0.05.
 2. The cathode active materialaccording to claim 1, wherein y is 0.0005≦y<0.005.
 3. The cathode activematerial according to claim 1, wherein y is 0.001≦y<0.005.
 4. Thecathode active material according to claim 1, wherein M is Mg, and z is0<z≦0.05.
 5. The cathode active material according to claim 4, wherein zis 0.005≦z≦0.02.
 6. A cathode for a nonaqueous electrolyte batterycomprising a cathode active material according to claim
 1. 7. Anonaqueous electrolyte battery comprising a cathode for a nonaqueouselectrolyte battery according to claim 6.